In various types of signal and control systems, such as railroad cab signaling systems, the actual speed of moving railway vehicles, is determined by a frequency generator which is driven by the wheels of the vehicles. In practice, the wheel driven generator produces a distorted sinusoidal alternating current signal having a frequency proportional to the speed on the moving vehicle. At first glance, it would appear plausible to simply increase the amplitude of the picked up signals by employing a high gain amplifier and clipping to produce a square wave signal which can then be differentiated to obtain a pulse signal. One of the problems with such a high gain amplifier arrangement resided in the fact that noise and other spurious signals even though of small amplitude could result in false speed signal pulses which can result in the unsafe operation of the railway vehicles. Another method of processing the picked up alternating current signals would be to utilize a switching circuit, such as a Schmitt trigger for squaring the sinusoidal waves. It will be appreciated that Schmitt trigger switching circuits are subject to the loss of hysteresis resulting in high gain due to various component failures which allow noise pulses to cause erratic and unsafe operation. Hence, it is essential that a pulse generating circuit which is intended to be used in a vehicle speed measuring operation must have high degree of immunity to noise and other spurious signals which are ever present in a cab signaling system. Since the number of generated electrical pulses is proportional to the actual speed of the moving vehicle, it is vital to ensure noise signals do not adversely effect the speed measuring ability of the cab signaling apparatus. That is, random noise or transient pulses should not be capable of changing the pulse rate of the signal generating circuit so as to cause the production of an erroneous speed measurement. Thus, an acceptable speed pulse generating circuit for cab signaling equipment must be highly insensitive to noise in order to provide the highest degree of safety to individuals utilizing the system. In addition, the security of the overall cab signaling system is predicated upon the fail-safe design of the apparatus and the high reliability of the operation of the apparatus.